Devil May Cry 5 walkthrough/SM10

Location
This Secret Mission is located at Mission 14: Diverging Point: V, after the area where the player fights the Mirage Goliath, Mirage Artemis and Miraggio Angelo, there will be a Blood Clot that must be destroyed in order to progress. Once it's destroyed, walk forward, past it, rather than go back and finish the mission. The glowing point will be in that area, then simply turn the camera around and find the pattern.

Walkthrough
The easiest way to complete this Secret Mission is by purchaing the "Promotion" move which allows V to ride Nightmare. The player spawns with the perfect amount of Devil Trigger to call the creature, then simply walk towards the Blue Orb fragment.